A significant number of human craniofacial syndromes are the result of abnormal development of the first (mandibular) branchial arch (BA1). Most of BA1 mesenchyme is derived from cephalic neural crest cells (NCC) that emerge from the lateral edges of the neural folds, migrate ventrally in close proximity to the developing pharynx, and populate BA1 primordia. At the level of BA1, NCCs receive signals from the local environment that regulate their proliferation, survival, and differentiation. Multiple developmental pathways are involved in regulating morphogenesis of BA1, with bone morphogenetic proteins (BMPs) playing a prominent role. Twsg1 is a secreted protein that binds to BMPs and modulates their activities. We have shown that Twsg1-deficient mice have abnormal morphogenesis of BA1, manifesting as agnathia, with variable degrees of accompanying craniofacial defects. Twsg1 mutant mice also show defects in foregut endoderm suggesting that Twsg1 signaling is necessary for foregut development. Our long-range goal is to understand molecular mechanisms underlying human craniofacial syndromes. The objective of the present study is to characterize the role of Twsg1 in mandibular arch morphogenesis. Our central hypothesis is that Twsg1 is required for morphogenesis of the medial region of BA1 and that foregut endoderm deficiency in Twsg1-/- mice affects patterning of cephalic NCC. The rationale for the proposed research is that knowledge obtained from this study will promote our understanding of the molecular and cellular basis of BA1 patterning that can ultimately lead to better diagnosis of human craniofacial syndromes, which result from abnormal development of BA1. Our studies will also inform how precise regulation of BMP signaling affects complex patterning events, such as those required for mandibular development. The following specific aims will be pursued: 1) Examine cephalic NCC formation and migration and mandibular arch morphogenesis in Twsg1-/- mice by using NCC- and BA1-specific markers, as well as assays of proliferation and apoptosis;2) Identify genes regulated by Twsg1 signaling in BA1 development by using microarray technology;3) Identify the cells that are the primary target of Twsg1 gene action in patterning of BA1 by generating endoderm-specific deletion of Twsg1;4) Understand how excess of Twsg1 affects molecular boundaries of BMP signaling during BA1 morphogenesis by implanting Twsg1-soaked beads into mouse mandibular explants. Lay summary: Abnormal development of the lower jaw is a common finding in children with various craniofacial syndromes. We have identified a gene, Twsg1, that is important for jaw development in mice. Since Twsg1 gene is also present in humans and lower jaw development is similar in mice and humans, this work can lead to better understanding and ultimately diagnosis of human craniofacial syndromes.